Next week's festive episode is historic for so many reasons, as it's a swansong for Peter Capaldi and head writer Steven Moffat as well as the first appearance of Jodie Whittaker as the Thirteenth Doctor.

Not to mention David Bradley returns as The First Doctor and Mark Gatiss joins Pearl Mackie in the TARDIS team as a beleaguered World War I soldier.

Gatiss offered a preview of 'Twice Upon a Time' on The One Show on Tuesday (December 19), drawing comparisons between the festive instalment and the 1973 multi-Doctor serial 'The Three Doctors'.

"It's a beautiful story, I think," Gatiss answered. "It's very touching, very funny. It's Steven [Moffat's] last one. Peter Capaldi's last one. Jodie Whittaker's first one. David Bradley is back as the First Doctor.

"It's like a sort of new 'Three Doctors'. For me, as a lifelong fan, to be in a regeneration story is amazing. To be at the end of so many eras, it's just fantastic."

In 'The Three Doctors', the Second and Third Doctors (Patrick Troughton and Jon Pertwee) were guided by the First Doctor (William Hartnell) in a quest to stop the destruction of Gallifrey by the rogue Timelord Omega (Stephen Thorne). Watch a classic clip below:

Much like in the '70s serial, it's safe to assume that the First and Twelfth Doctors will definitely butt heads as they both try to stave off regeneration in 'Twice Upon a Time' next week.